DNA damage response networks are common targets for carcinogenic mutations and may be excellent targets for response-modifying drugs. The central core of these networks consists of phosphatidyl inositol 3-kinase-related kinases (PIKKs), e.g. human Atm and Atr, which activate downstream pathways in response to DNA damage. PIKKs phosphorylate components of complexes that are recruited to damageinduced foci, including the Mrell/Rad50/Nbsl complex, 53BP1, and Brcal, and activate effector kinases Chkl and Chk2 that amplify the signal and help convey it to effector proteins. In budding yeast, the PIKK Mecl (Atm/Atr ortholog) is coupled to protein kinase Rad53 (Chk2 ortholog) through Mecl-dependent phosphorylation of the "mediator" Rad9 (unrelated to human Rad9). Mammalian BRCA1, 53BP1, and now MDC1/NFBD1 are candidate Rad9 orthologs that resemble Rad9 in having twin carboxyl terminal BRCT domains. Like Rad9, they undergo PIKK-dependent phosphorylation, form damage-induced complexes with other checkpoint proteins and are recruited to sites of DNA damage. MDC1/NFBD1 is intimately involved in DNA damage response mechanisms. The PIKK-Mediator-Chk core of DNA damage signaling will be investigated using both the simpler yeast cascade Mecl to Rad9 to Rad53, and the more complex interactions of ATM/ATR in regulation of NFBD1. Aim 1. A combination of biochemical and molecular methods will be used to determine the mechanism through which the mediator Rad9 and the PIKK Mecl regulate Rad53. Aim 2.. siRNA oligonucleotide knockdowns will be used to identify unique and overlapping functions of BRCA1, 53BP1, and NFBD1 in DNA checkpoint responses. Aim 3. Molecular techniques will be used for functional analysis of NFBD1/MDC1 in the activities identified in Aim 2.